High pressure sodium arc lamps have been in commercial production for many years and have been subject to many improvements in design, materials and processing. Such lamps include a translucent ceramic arc tube, an outer envelope including an electrical connector, and a frame for supporting the arc tube within the outer envelope. The frame is electrically conductive and carries power to the arc tube. The arc tube is typically fabricated from polycrystalline alumina or yttria and contains an amalgam of mercury and sodium for producing light having a desired output spectrum. Tungsten or molybdenum electrodes are positioned within the arc tube at opposite ends and are attached to feedthroughs selected to have thermal expansion characteristics closely matched to those of the ceramic arc tube. The feedthroughs are hermetically sealed in openings at opposite ends of the arc tube. Niobium, usually containing about 1% zirconium by weight, is the preferred feedthrough material for alumina arc tubes.
A variety of electrode feedthrough structures and techniques for hermetically sealing the feedthroughs to the arc tube are known in the art. In one commonly used structure, the feedthrough is a niobium tube. A tungsten coil electrode is attached to the niobium tube by a tungsten electrode support rod. The opening in each end of the arc tube is sufficiently large for insertion of the electrode and the niobium tube. An insert button is sintered directly into the end of the arc tube, and a ceramic sealing button or ring is sealed with a low melting point ceramic frit to the end of the arc tube over the feedthrough to extend the length of the seal and to improve its reliability. Such a structure is disclosed in U.S. Pat. No. 4,539,511. An alternative construction utilizes an arc tube without an insert. A sealing button having a groove to aid in flow of the sealing material is disclosed in U.S. Pat. No. 4,713,580 issued Dec. 15, 1987 to Schoene. An arc tube construction which utilizes a connection wire to position a feedthrough tube in the arc tube during sealing is disclosed in U.S. Pat. No. 4,804,889 issued Feb. 14, 1989 to Reid et al.
It is known in the prior art to construct high pressure sodium arc lamps so that the interior of the electrode feedthrough tube and the discharge region in the ceramic arc tube are interconnected by a passage of sufficient cross section to permit flow of the vaporized fill material. The interior of the feedthrough tube typically operates at a lower temperature than the arc tube. Therefore, the fill material tends to condense in the feedthrough tube. This construction is commonly referred to as an external reservoir arc tube, since the fill material condenses in a region external to the discharge region. External reservoir construction is disclosed in U.S. Pat. No. 4,342,938 issued Aug. 3, 1982 to Strok, European Patent Application No. 0,225,944 published Jun. 24, 1987, U.S. Pat. No. 4,827,190 issued May 2, 1989 to Masui et al, European Patent Application No. 0,265,266 published Apr. 27, 1988, U.S. Pat. No. 4,035,682 issued Jul. 12, 1977 to Bubar and U.S. Pat. No. 4,065,691 issued Dec. 27, 1977 to McVey. The external reservoir arc lamp construction is believed to provide lower sodium loss than conventional arc lamps and to provide a more constant level of light output over the life of the arc lamp.
In developing an external reservoir, high pressure sodium arc lamp suitable for high volume production, a number of practical requirements must be met. The arc lamp assembly including the electrode feedthrough having an external reservoir must have a low manufacturing cost. The electrode feedthrough tube which defines the external reservoir must be hermetically sealed at its outer end and must have a passage at or near its inner end interconnecting the interior of the tube and the discharge region within the arc tube. The passage must be of sufficient cross-section to permit flow of vaporized fill material, but must not be so large as to permit droplets of fill material to pass from the external reservoir into the discharge region. In addition, the rod which supports the tungsten electrode must have a reliable connection, both electrically and mechanically, to the electrode feedthrough tube. Finally, the external portion of the feedthrough tube must be electrically connected to the lamp frame.
These requirements have been addressed in various ways in the prior art. However, none have proven entirely satisfactory. For example, aforementioned U.S. Pat. No. 4,827,190 and European Patent Application No. 0,225,944 disclose a feedthrough construction wherein the feedthrough tube is hermetically sealed by fusing the outer end of the tube. This technique adds an extra step to the lamp fabrication process and requires a heat absorbing plate to be used during the fusing process. In addition, there is a risk that the fused end will leak. The aforementioned U.S. Pat. No. 4,342,938 discloses a construction wherein the external end of the feedthrough tube is crimped and welded to provide hermetic sealing. The possibility of a leak also exists in this construction.
In the above patents, an aperture is provided in the wall of the feedthrough tube to interconnect the interior of the tube and the discharge region. An additional step is required to form the aperture. In European Patent Application No. 0,265,266, the electrode feedthrough tube is flattened against the electrode support rod by mechanical deformation, leaving capillary passages between the interior of the feedthrough tube and the discharge region. The disclosed configuration has four lobes in the deformed portion of the feedthrough tube. A disadvantage of the disclosed configuration is that portions of the tube in the deformed region often extend beyond the outside diameter of the feedthrough, thereby making it impossible for the feedthrough tube and the electrode to be inserted through the opening in the ceramic arc tube during assembly.
It is a general object of the present invention to provide improved ceramic arc tube assemblies for high pressure discharge lamps.
It is another object of the present invention to provide improved electrode feedthrough assemblies for high pressure discharge lamps.
It is still another object of the present invention to provide ceramic arc tube assemblies which are low in cost and easy to manufacture.
It is a further object of the present invention to provide improved methods for manufacturing ceramic arc tube assemblies.